1. Field of the Invention
The invention of the present application relates to a thin film deposition apparatus that produces a thin film on the surface of a semiconductor wafer substrate, and more specifically it relates to a selective deposition technique that selectively deposits a thin film only in specific regions of the substrate surface.
2. Discussion of Related Art
The deposition of thin films on the surface of semiconductor wafer substrates is frequently performed in the manufacture of various electronic devices. In particular, in the manufacture of integrated circuits such as LSIs, thin films are selectively deposited only on specific regions of the surface of a substrate. For example, a process is sometimes performed wherein a wiring pattern is formed in an insulating film of silicon oxide (SiO.sub.2) or silicon nitride (Si.sub.3 N.sub.4) on a silicon substrate, and a silicon film is selectively deposited only on the regions of the substrate surface where the silicon is exposed.
FIG. 5 is a front view outlining the configuration of a conventional thin film deposition apparatus used for this sort of selective deposition of silicon. The thin film deposition apparatus shown in FIG. 5 has a process chamber 1 equipped with pumping systems 11 and 12, and a gas introduction means 2 that introduces a process gas into process chamber 1. A susceptor 3 on which a substrate 9 is positioned and a heater 4 which heats substrate 9 are disposed inside process chamber 1.
The apparatus shown in FIG. 5 is a cold-wall apparatus in which the enclosure walls of process chamber 1 are fitted with a cooling mechanism (not illustrated). A first pumping system 11 which pumps down the whole interior of process chamber 1, and a second pumping system 12 which principally pumps down the region surrounding heater 4 are also provided. First and second pumping systems 11 and 12 both employ ultra-high vacuum pumping systems using turbo-molecular pumps.
Gas introduction means 2 is made to introduce disilane (Si.sub.2 H.sub.6)--a gaseous silicon hydride--as the process gas.
Susceptor 3 is shaped into a block which is fixed to the bottom surface of process chamber 1, and substrate 9 is mounted on its upper surface. A lift pin 5 which can be raised and lowered is provided in the interior of susceptor 3. Lift pin 5 rises and falls through a hole provided in the upper surface of susceptor 3. When mounting a substrate 9 on susceptor 3, lift pin 5 is raised up so that it projects from the upper surface of susceptor 3, and lift pin 5 is lowered after the substrate 9 has been mounted on top of lift pin 5. Substrate 9 is thereby mounted on the upper surface of susceptor 3. Susceptor 3 is formed from a material such as silicon, graphite or SiC (silicon carbide), and is made so that it contacts substrate 9 with good thermal conductivity.
A heater 4 is disposed inside susceptor 3. A heater 4 that heats substrate 9 mainly by radiative heating is used. Specifically, a carbon heater that emits heat by conducting electricity can be used. The heat radiated from heater 4 is conferred to susceptor 3, and substrate 9 is heated via susceptor 3. The temperature of substrate 9 is sensed by a thermocouple (not illustrated) and is sent to a heater control unit (not illustrated). The heater control unit performs feedback control of heater 4 according to the sensed result, whereby the temperature of substrate 9 is kept at a set temperature.
Susceptor 3 is made of the same silicon as substrate 9 to avoid contamination of substrate 9. To avoid contamination of the atmosphere inside process chamber 1 by the release of occluded gas from heater 4 when it becomes hot, second exhaust system 12 pumps down the region surrounding heater 4.
A cooling mechanism (not illustrated) is also provided at the side parts of susceptor 3. This is to prevent process chamber 1 from being heated by the conduction of heat from susceptor 3 to process chamber 1.
A heat-reflecting plate 6 is positioned above the substrate 9 mounted in susceptor 3 so as to be parallel with substrate 9. Heat-reflecting plate 6 reflects the radiation emitted from substrate 9 and susceptor 3 and returns it to substrate 9, thereby improving the efficiency with which substrate 9 is heated.
Heat-reflecting plate 6 is made of silicon. By making heat-reflecting plate 6 from the same kind of material as the film deposited on the surface of substrate 9, the thin film deposited on the surface of heat-reflecting plate 6 can be prevented from peeling away.
The silicon film deposited by thermal decomposition of a gaseous silicon hydride compound as described below is deposited not only on the surface of substrate 9 but also on heat-reflecting plate 6. If heat-reflecting plate 6 is made of a completely different material other than silicon, the thin film will have poor adhesion and can easily peel away due to internal stress. Parts of the film that peel away will form globular dust particulates that float about inside process chamber 1. If these particulates adhere to the surface of substrate 9, they will give rise to defects caused by localized reduction of the layer thickness, which are a cause of faulty products. To prevent the thin film from peeling away, heat-reflecting plate 6 uses the same silicon material as the thin film being formed.
The operation of a conventional apparatus relating to the above configuration is described next.
A substrate 9 is transferred into process chamber 1 via a gate valve 13, and is mounted on susceptor 3 by raising and lowering lift pin 5. The interior of process chamber 1 is pumped down in advance to 10.sup.-8 Torr or thereabouts by first and second pumping systems 11 and 12.
Heater 4 is operated before the film deposition begins, and the substrate 9 mounted on susceptor 3 is heated by the heat from heater 4 and maintained at the desired temperature after reaching thermal equilibrium. After this state has been achieved, gas introduction means 2 is operated and a gaseous silicon hydride compound is introduced into process chamber 1 as the process gas. The process gas diffuses inside process chamber 1 and arrives at the surface of substrate 9. The gaseous silicon hydride compound then decomposes under the heat at the surface of substrate 9, whereby a film of polycrystalline silicon is deposited at the surface.
The surface of substrate 9 has a wiring pattern formed in an insulating film of silicon oxide or silicon nitride, so that the surface contains regions of exposed silicon--the material of substrate 9--and regions where silicon oxide or silicon nitride is formed on the surface. The thermal decomposition reaction rate at the silicon surface is much higher than the thermal decomposition reaction rate at the silicon oxide film surface or silicon nitride film surface. The silicon film is thus selectively deposited only on the silicon surface. Selective deposition of silicon is thereby achieved.
FIGS. 6(1) and (2) show the results of experimental selective deposition of silicon using the conventional apparatus shown in FIG. 5. Specifically, FIGS. 6(1) and (2) show photographs of the reflection high energy diffraction (RHEED) pattern observed in film deposition using the apparatus of FIG. 5 with the temperature of substrate 9 held at 700.degree. C. and with disilane introduced at 6 sccm. FIG. 6(1) shows the state 30 seconds after introduction of the process gas, and FIG. 6(2) shows the state after 300 seconds.
As shown in FIG. 6(1), in the state 30 seconds after introducing the gas, the pattern contains a mixture of bright vertically-extending parts and a diffuse region of brightness. The bright vertically-extending parts represent the diffraction spots from the crystalline lattice, indicating the presence of crystalline silicon at the surface of substrate 9.
On the other hand, the diffuse region of brightness represents the reflection of electrons with no periodicity from disordered (amorphous) crystals. In this case, the electrons are reflected uniformly over a wide angle, resulting in the diffuse region of brightness shown in FIG. 6(1). The halo means that amorphousness is present at the surface of substrate 9. In this case, it is considered that the surface of the silicon oxide film or silicon nitride film is already exposed at the surface of substrate 9, and that the halo observed in FIG. 6(1) is observed due to the reflection of electrons at the surface of the silicon oxide film.
As shown in FIG. 6(2), in the state 300 seconds after starting to introduce the gas, a ring-shaped diffraction pattern is observed in addition to the diffraction spots that were also seen in FIG. 6(a). This ring-shaped diffraction pattern is symptomatic of the deposition of a polycrystalline silicon film at the amorphous surface of the silicon oxide film. This ring-shaped diffraction pattern increases sharply in intensity during the interval between 300 and 330 seconds. Accordingly, it is considered that about 330 seconds after starting to introduce the gas, the polycrystalline silicon film covers not only the silicon surface but also the surface of the silicon oxide film.
FIGS. 7(1) and (2), like FIGS. 6(1) and (2), show the results of experimental selective deposition of silicon using the conventional apparatus shown in FIG. 5. Specifically, FIGS. 7(1) and (2) show photographs taken with a scanning electron microscope (SEM) during film deposition using the apparatus of FIG. 5 with the temperature of substrate 9 held at 700.degree. C. and with disilane introduced at 6 sccm as in FIGS. 6(1) and (2). FIG. 7(1) shows the results observed on a substrate 9 removed from process chamber 1 60 seconds after starting to introduce the process gas, and FIG. 7(2) shows the results observed on a substrate 9 removed from process chamber 1 330 seconds after starting to introduce the process gas.
As shown in FIG. 7(1), in the state 60 seconds after starting to introduce the gas, a deposit of about 70 nm thickness is observed at the surface of the silicon. This is the epitaxial silicon layer deposited at the silicon surface. Also, no deposit is observed at the surface of the silicon oxide film. Meanwhile, as FIG. 7(2) shows, in the state 330 seconds after starting to introduce the gas a substantial polycrystalline silicon film is deposited at the surface of the silicon oxide film.
As the above results show, in the selective epitaxial deposition of silicon under the above conditions, the selective growth of silicon continues when 60 seconds have elapsed, whereas the conditions for selective epitaxial growth break down after 300 seconds have elapsed and a polycrystalline silicon film is also deposited on the surface of the silicon oxide film. This is because, although the reaction rate is much slower than on the silicon surface, a prolonged gas introduction time leads to a large cumulative quantity of gas supplied, and the disilane decomposition reaction will thus also occur on the silicon oxide film. Nonetheless, a epitaxial silicon film can still be deposited only on the silicon surface by precisely controlling the gas introduction time.
However, in the abovementioned conventional apparatus, there are exposed positions inside process chamber 1 that are similarly heated to a high temperature, like the surface of substrate 9. For example, the heat-reflecting plate 6 provided opposite substrate 9 to increase the heating efficiency receives radiated heat from substrate 9 and susceptor 3 and is heated to a high temperature in the same way as substrate 9. For example, when the distance between substrate 9 and heat-reflecting plate 6 is about 45 mm, the pressure is about 10.sup.-3 Torr, or less, and substrate 9 is heated to about 600.degree. C., and the heat-reflecting plate 6 is also heated to about 265.degree. C.
Since the process gas diffuses around the interior of process chamber 1, a thin film is deposited at the surfaces of parts that have thus been heated to a high temperature in the same way as the deposition on substrate 9. If the heated positions inside process chamber 1 are made of a material other than silicon, then the deposition of thin films should in principle be suppressed by a similar mechanism to that of the selective deposition on substrate 9. However, during repeated film deposition processes, the cumulative quantity of process gas supplied to the heated positions will suddenly exceed the threshold beyond which the abovementioned conditions for selective deposition break down, and thin films will also start to be deposited at the heated positions. This deposition of thin films at heated positions inside process chamber 1 gives rise to the following two problems.
The first problem is that the deposition of a thin film alters the radiation reflection conditions, causing a change in the heating conditions of substrate 9 as a result. For example, although the parts situated inside process chamber 1 are often made of stainless steel, the deposition of a silicon thin film on the surface of this stainless steel will affect the radiation reflectivity. As a result, the amount of radiation reflecting off the wall surfaces and returning to substrate 9 will change and the repeatability of temperatures attained with the same electrical power will be made less precise.
Since the reflectivity is normally reduced by the deposition of a thin film, the power supplied to heater 4 must be increased to compensate for this reduction of reflectivity. However, controlling the supplied power in this way necessitates sensing the temperature of substrate 9 with high precision and applying feedback to heater 4, which requires a very rigorous control precision.
The second problem is that particulates can be produced by parts of the deposited thin film peeling away, thereby impairing the quality with which substrate 9 is processed. If the thin films deposited at heated positions inside process chamber 1 reach this thickness, they can peel away either under their own weight or due to internal stress. Although heat-reflecting plate 6 suppresses the peeling of thin films to some degree since it is made of the same silicon material as substrate 9, as mentioned above, a silicon thin film is deposited in the same way as on substrate 9 since these positions have the same material as substrate 9 at their surface, and so the cumulative amount of deposition is higher than at other heated positions and peeling can easily occur due to internal stress.
Attempts have been made to make improvements by thermally oxidizing the surface of heat-reflecting plate 6 to cover it with a thermal oxide film and suppress the deposition of a silicon film on silicon heat-reflecting plate 6. If there is a film of silicon oxide at the surface, then this should suppress the deposition of a silicon film by the same mechanism as the selective deposition on substrate 9.
However, when a film is deposited on substrate 9 with, for example, a substrate temperature of 600.degree. C. and a disilane flow rate of 12 sccm, the deposition of a thin film of silicon on the surface of heat-reflecting plate 6 was observed after performing film deposition processes on at most 5 substrates. Having completed film deposition on about 4000 substrates, it was confirmed that the film deposited on heat-reflecting plate 6 had peeled away, resulting in about 60 dust particulates adhering to the surface of substrate 9. These results show that even if the surfaces of parts made of silicon are covered with a film of silicon oxide, the cumulative amount of gas supplied can still exceed the conditions for selective deposition so that a silicon film is deposited.
In conventional apparatus, to suppress the problem of dust particulates generated due to peeling of thin films in this way, maintenance is performed after the film deposition process has been repeated a fixed number of times, in which the interior of process chamber 1 is opened to the atmosphere and the parts inside process chamber 1 are replaced. Process chamber 1 is then pumped down to an ultra-high vacuum, and the processing is restarted after confirming that there are no dust particulates and that the process can be performed repeatably. To perform this sort of maintenance, the apparatus must be taken out of service for 6 hours and worked on by 3 operators, requiring a total of 18 man-hours of labor. Consequently, this can seriously impair the productivity of the apparatus.